The continuing evolution in the medical field of the study and control of cardiac activity requires advances in the designs of implantable devices utilized to monitor and control the cardiac activity. Various types of electrodes are currently utilized which are either implanted into an interior chamber of the heart or affixed to the exterior surface of the heart. These devices, depending upon their intended locations, have associated specific design requirements. For the devices designed to be affixed to the exterior surface of the heart, the anatomical environment places design on both the materials and construction of the devices. For example, cardiac defibrillator patch leads preferably have a relatively large electrode surface area in order to allow delivery of a polarizing charge or pacing stimulation of sufficient magnitude.
For patch electrodes which are affixed to the external surface of the heart, the environment requires that the electrode be extremely resistant to fractures caused by the flexing resulting from the continuous beating of the heart. In addition, the electrodes must exhibit high conductivity and polarizing capacity, high flexibility, and biological inertness. For cardiac defibrillation electrodes, the use of extremely fine titanium or platinum wire mesh, which provides very good electrical properties while conforming to the shape of the heart, has become standard as the material for a defibrillator patch electrode.
Additionally, when using electrodes which are placed directly on the heart, it is a requirement that the electrical conductors interconnecting the patch electrode and a signal processing and power generating assembly be extremely flexible and resistant to fracture from repeated flexion. As may be appreciated, the heart undergoes continuous movement which cannot be inhibited by the conductors or the electrodes affixed to the surface of the heart. In recognition of the critical nature of the devices, any fracture or degradation in the performance of the electrode and/or the conductors is unacceptable.
Given the above constraints for both the defibrillator patch electrode and the associated electrical conductors, an optimum design may be a conductor which flares at one end into a mesh which may be affixed to the surface of the heart. However, this configuration would be difficult to manufacture, and it is often the case that the wire forming the electrode patch is of a different material from that of the conductor. Accordingly, some mechanism for affixing the electrode patch to the conductor is required. One such arrangement is depicted and described in U.S. Pat. No. 4,314,095 (Mirowski et al.). This patent utilizes a generally U-shaped sleeve which is affixed to the electrode mesh and which is designed to form a channel into which the flexible electrical conductors are inserted and crimped into place.
The various types of materials for the electrodes and conductors, as well as various specific design considerations influence the method of affixing the conductors to the wire mesh of the electrode patch. In this regard, it should be noted that the particular difficulty comes in providing secure electrical and mechanical contact between the electrical conductors and electrode mesh. This point of joinder is one of the more challenging aspects of the design of the defibrillator patch lead.